Looks fairly clean and these burn over time as the contacts go bad.
Replace the plugs --
8 pin plug = CS156-08-LR or 09-50-8081
10 pin plug = CS156-10-LR or 09-50-8101
20 pin plug = CS156-20-LR
Crimp contacts -- use 08-52-0113
Keys (one per connector) -- 15-04-0219 for CS156 plugs or 15-04-0297 for 09-50 plugs
Headers -- 26-48-1241

Make sure fuse holder at F5 isn't bad. If so, use high current fuse clips such as 1A1907-03's.
Can find these at:
www.GreatPlainsElectronics.com

Little bit of info on this board at pinwiki:
http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Rectifier_Boards

Ed

Typical failure and you do not have to replace the entire rectifier board if you are able to desolder/solder new header pins. Remember you have to resolder the transformer wires if you replace the entire board, so soldering work to be done either way.

You will need a crimp tool as well. http://www.greatplainselectronics.com/Category-138.asp I recommend BCT-1.

It is not a pain in the ass, but requires correct tools and some practice. Watch some youtube videos on soldering and get some junk boards to practice on. Probably a good idea to replace them all. If they have any burn marks / tarnish they must go.

Radio shack even sells a very usable desoldering iron for less than $20. The tips on them go to hell really fast so if you go that route pick up some extra tips.

You can also buy a completely new board that you have to solder a dozen or so wires onto for $68 shipped or a no solder version for a little more. I went the $68 route due to my board being in worse condition than yours and did not mind the soldering part as I had the tools already.You will still need the crimping tool but it comes with the connectors and crimps.Pinlogic is who I got mine from..Just food for thought.

This site in invaluable for early Bally SS machines:
http://techniek.flipperwinkel.nl/ballyss/rep/index1.htm#minimum

This guy sells nice kits I've used to repair my AS-2518-18s before:
http://bigdaddy-enterprises.com/index.htm